Re: [BUG] cpu-hotplug: Can't offline the CPU with naughty realtime processes

[Srivatsa Vaddagiri <vatsa@in.ibm.com>]

On Tue, May 08, 2007 at 04:16:06PM +0900, Satoru Takeuchi wrote:
> Sometimes I wonder at prio_array. It has 140 entries(from 0 to 139),
> and the meaning of each entry is as follows, I think.
> 
> +-----------+-----------------------------------------------+
> | index     | usage                                         |
> +-----------+-----------------------------------------------+
> | 0 - 98    | RT processes are here. They are in the entry  |
> |           | whose index is 99 - sched_priority.           |

>From sched.h:

/*
 * Priority of a process goes from 0..MAX_PRIO-1, valid RT
 * priority is 0..MAX_RT_PRIO-1, and SCHED_NORMAL/SCHED_BATCH
 * tasks are in the range MAX_RT_PRIO..MAX_PRIO-1.

so shouldn't the index for RT processes be 0 - 99, given that
MAX_RT_PRIO = 100?

> +-----------+-----------------------------------------------+
> | 99        | No one use it? CMIIW.                         |
> +-----------+-----------------------------------------------+
> | 100 - 139 | Ordinally processes are here. They are in the |
> |           | entry whose index is (nice+120) +/- 5         |
> +-----------+-----------------------------------------------+
> 
> What's the purpose of the prio_array[99]? Once I exlore source tree
> briefly and can't found any kernel thread which uses this entry.
> Does anybody know?

-- 
Regards,
vatsa